What is a word that means a Roman commander of 100 men?

Centurion

it used to be a hundred men but over time it fell to 80 men to a century,which by the late first century AD would have been made up like this:

MEN TOTAL

6 centuries of 80 men 480 {one cohort}

10 cohorts made up a legion thus:

9 cohorts of 480 men 4,320 men

The first cohort was double sized and made up of ten centuries of 800 men

800 + 4,320= 5,120 men